The subject matter disclosed herein generally relates to heat exchangers and, more particularly, to heat exchangers having coatings thereon.
Heat exchangers are widely used in various applications, including but not limited to heating and cooling systems including fan coil units, heating and cooling in various industrial and chemical processes, heat recovery systems, and the like, to name a few. Many heat exchangers for transferring heat from one fluid to another fluid utilize one or more tubes through which one fluid flows while a second fluid flows around the tubes. Heat from one of the fluids is transferred to the other fluid by conduction through the tube walls. Many configurations also utilize fins in thermally conductive contact with the outside of the tube(s) to provide increased surface area across which heat can be transferred between the fluids, improve heat transfer characteristics of the second fluid flowing through the heat exchanger and enhance structural rigidity of the heat exchanger. Such heat exchangers include microchannel heat exchangers and round tube plate fin (RTPF) heat exchangers.
One of the primary functions of a heat exchanger is to transfer heat from one fluid to another in an efficient manner. Higher levels of heat transfer efficiency allow for reductions in heat exchanger size, which can provide for reduced material and manufacturing cost, as well as providing enhancements to efficiency and design of systems that utilize heat exchangers such as refrigeration systems. However, there are a number of impediments to improving heat exchanger system efficiency.
For example, many metal alloys used for heat exchanger construction such as aluminum alloys are subject to corrosion. Applications located in or close to marine environments, particularly, sea water or wind-blown seawater mist create an aggressive chloride environment that is detrimental for these heat exchangers. This chloride environment rapidly causes localized and general corrosion of braze joints, fins, and refrigerant tubes. The corrosion modes include galvanic, crevice, and pitting corrosion. Corrosion impairs the heat exchanger ability to transfer heat via several mechanisms including loss of structural integrity and thermal contact with refrigerant tubes. Corrosion products also accumulate on the heat exchanger external surfaces creating an extra thermal resistance layer and increasing airflow impedance. In addition, corrosion eventually leads to a loss of refrigerant due to tube perforation and failure of the cooling system. Polymer coatings are often used to protect heat exchanger surfaces from corrosion and physical damage. Many polymers, however, are inefficient conductors of heat, and their use as a protective coating can adversely affect heat transfer efficiency.
Additionally, heat exchangers used as evaporators in refrigeration systems are often subject to the formation of frost on the exterior surface of components of the heat exchanger such as heat exchanger fins and tubes. Frost on these heat exchanger surfaces adversely affects heat transfer efficiency by reducing heat transfer, which adversely affects the overall efficiency of the refrigeration system. Frost formation is often addressed by operating the refrigeration system in a defrost cycle, which further reduces system efficiency. Such adverse impacts on the refrigeration system often require the heat exchanger and other system components to be designed for larger capacity, leading to increased system cost and complexity, in addition to the increasing operating costs to meet system performance requirements.
In view of the above and other issues, there continues to be a need in the art for new approaches to heat exchanger design and manufacture.